In the case of a trench (groove) gate type power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) formed in a structure incorporating, for example, a p-type epitaxial layer as an upper layer of an n+type substrate, it is known to reduce the risk of punch-through breakdown of the trench gate by forming an n-type drain region extended between the n+type substrate and the bottom of a trench, and by forming a junction of the n-type drain region and the p-type epitaxial layer extended between the ntype substrate and a partition wall of the trench. This is illustrated in, for example, Japanese Patent Laid-Open No. 2000-164869.
It is also known to improve switching loss of the trench gate power MOSFET by providing an n−type epitaxial layer doped to have a first conductivity type, and a well layer doped to have a second conductivity type, over a semiconductor substrate heavily doped to have the first conductivity type, thereby providing a deep trench gate, isolated by an insulating layer, inside an upper side layer made up of the n−type epitaxial layer and the well layer, and thereby providing a drain region of high conductivity under the trench gate. It is further known to provide source regions heavily doped to have the first conductivity type, adjacent to the trench gate, and to provide a main body region doped more heavily than the well layer, in the upper part of the well layer, to thereby reduce ON-resistance of the drain region. This is illustrated, for example, in Japanese Patent Laid-Open No. 2000-299464.
A power transistor, such as the power MOSFET, may be used for high power applications, such as wherein the power is not less than several watts. One such power transistor, a power MISFET, may be a vertical type and/or a lateral type MISFET, depending on the structure of the gate thereof. A MISFET may be further classified as a trench (groove) gate type, or a planar gate type.
A power MISFET may be used as a switching element of a DC-DC converter, for example, such as for use as a power source circuit for a computer. Since there has recently been a trend of larger current requirements for a CPU (Central Processing Unit) of a computer, to which the DC-DC converter supplies power, a larger current is required of the DC-DC converter as well. Requiring larger currents may cause an increase in ON-loss for a power MISFET. Accordingly, lower ON-resistance also is requited of a power MISFET used in a DC-DC converter.
In a trench gate type power MISFET, a deep groove is formed in a top, element-forming surface of a semiconductor substrate (hereinafter referred to as a substrate), and a gate is formed by embedding a conductor in the groove. Further, in a planar gate type power MISFET, a gate is formed over a top surface of a substrate through the intermediary of a gate insulator. Accordingly, in the trench gate type power MISFET, a current flow path in the substrate becomes shorter as compared with a planar gate type power MISFET, so that ON-resistance can be reduced. However, since a gate insulator is formed on the sidewalls and bottom surface of the groove, there may be an increase in capacitance due to the gate insulator acting as a capacitance insulator, and the gate part acting as a capacitance electrode, as the groove is rendered deeper. On the other hand, in a planar gate type power MISFET, capacitance from the gate insulator acting as the capacitance insulator and the gate acting as the capacitance electrode is reduced as compared with the trench gate type power MISFET, but the number of the gate parts that can be disposed per unit area is reduced if a sufficient gate length to prevent pinch off is present. Hence, if an area of a semiconductor chip, over which the planar gate type power MISFET is formed is constant, the number of the gate parts that can be disposed on the semiconductor chip is less than that in the case of a trench gate type power MISFET. As a result, in the case of the planar gate type power MISFET, ON-resistance of the semiconductor chip increases as compared with the trench gate type power MISFET. Hence, if the trend for the DC-DC converter to higher frequency and larger current continues, the known art cannot simultaneously meet lower ON-resistance and lower capacitance required of a power MISFET.
Thus, the need exists for a semiconductor device and method that provides a power MISFET that simultaneously provides lower ON-resistance and lower capacitance.